I'm thinking of getting either a P1 or a P53 for my next work upgrade. The P53 will likely have better thermals, but the P1 has an option to remove the discrete GPU, and just use the integrated GPU. As I have no need for a fancy GPU, I figure it would reduce unnecessary power draw.

Forgot to add - and you get a decent display, and it's portable, and it's still cheaper than MP.

And, as an added bonus, you can have major shoulder problems for life from lugging that 17" behemoth around.

Hah, I agree...But, on the other hand, you can carry it when needed, and I do regret not buying one of those for work instead of Razer Blade, "because it's thin and light" - it gets overly hot and noisy when doing heavier work and is definitely not up to par to these...

What operating system does it come with?

I think Lenovo (together with other questionable actions that alienate their userbase) removed the option to choose Linux for workstations, so it comes only with Windows 10. But Thinkpads are traditionally very well supported on various Linux distributions.